Radiation detection approaches exist that employ photosensors incorporating a microcell (e.g., a single photon avalanche diodes (SPAD)) operating in Geiger mode. Certain of these approaches have been implemented in large area devices, such as may be used in nuclear detectors. A readout pixel can be made up of an array of microcells, where each individual microcell can be connected to a readout network via a quenching resistor exhibiting resistance between 100 kΩ to 1 MΩ, known as solid state photomultiplier (SSPM), silicon photomultipliers (SiPM), multi-pixel photon counting (MPPC). When a bias voltage applied to the silicon photomultiplier (SiPM) is above breakdown, a detected photon generates an avalanche, the APD capacitance discharges to a breakdown voltage and the recharging current creates a signal.
Typically, the pulse shape associated with a single photo electron (SPE) signal has a fast rise time, followed by a long fall time. When detecting fast light pulse (e.g., on the order of tens of nanoseconds) such signals are aggregated across the numerous microcells forming a pixel of a SiPM device. The resulting pulse shape of the summed signal has a slow rise time (e.g., in the tens of nanoseconds) due to the convolution of single microcell responses with detected light pulse. Therefore, it is difficult to achieve good timing resolution with these devices due to the slow rise time of the aggregated signal for a given light pulse.
Analog SiPMs can have pixel outputs bonded-out by wires attached to the wafer, or by using short vertical interconnects implemented in Through-Silicon-Via (TSV) technology. Microcells can be connected by traces, and typically one or a few pads per array of microcells (pixel) can be used as output (wire bonds or TSV). An analog SiPM typically requires a front-end electronics to buffer (and/or amplify) the signal from the SiPM for further processing. Digital SiPM (dSiPM) technology has front-end electronics built-in to each of microcells to produce a digital output pulse. The microcells of a dSiPM communicate with an external controller having typically high clock speeds.
Due to the difference in actual position of microcells in an array, there can be a significant variation of time delay of pulse propagation across pixels. This variation degrades timing performance of the device. Attempting to equalize trace length by extending certain traces can significantly increase parasitics, and degrade signal pulse shape due to the limited driving capability of the microcell. Extending trace lengths or creating delays by incorporating additional circuits both require dedicating pixel space to these approaches, thus reducing the detector's active area.